Doppler sensors have become widely used and utilize various frequencies. At microwave frequencies, Doppler sensors are usually realized through the use of quadrature mixers. Microwave based sensors have inherently lower sensitivity (greater than micrometer displacements) than their optical counterparts. High frequency optical Doppler sensor has high sensitivity and lower interference with common consumer electronics. Further, high frequency optical Doppler sensor is more directional and provides for more compact structures. However, high-frequency optical sensors have several drawbacks, such as difficulty with alignment and diffraction loss due to surface roughness. Therefore, optical Doppler sensors are not desirable for applications involving the detection of complex objects. Furthermore, optical wavelengths cannot penetrate through many common materials such as fabrics, plastics and insulation.
As both low frequency microwaves and high frequency optics have advantages and disadvantages, certain systems have attempted to use the intermediate frequency spectrum between the low-frequency microwaves and the high-frequency optics. For example, quadrature mixer based mmW Doppler radar has been recently studied for remote monitoring of vital signs by the authors' group.